Reaction-injection-molding (RIM) is a well-known process for molding polymeric parts. In the RIM process, low molecular weight, reactive, low viscosity liquids are injected at a high pressure into a small mixing chamber and thence into a mold cavity. The liquid reactants polymerize in the mold to form the molded article. The RIM process is useful with a variety of polymer systems, but is most commonly used to form polyurethanes by the reaction between polyols and isocyanates. The reactants or prepolymers (i.e., polyol and isocyanate) are stored separately, and via appropriate high pressure pump metering units, are delivered as liquids at pressures between about 1500-3000 psi to a mixing head adjacent the mold. At the mixing head, the two liquids impinge and are thoroughly mixed before being injected into the mold cavity. Once in the mold cavity, the polymerization reaction completes itself and the finished article is formed. For polyurethanes, the pressure in the mold itself reaches about 100 to about 250 psi.
RIM molds, like many molds, comprise at least two segments which, in a mold-closed position, come together to define a mold cavity therebetween and into which the reactants are injected. In a mold-open position, the mold permits removal or ejection of the molded article from the mold cavity. The mold segments each have a face thereon confronting the other mold segment. The faces have complementary shapes and come together, in a mold-closed position, along a surface known as a parting line. One edge of the parting line is exposed to the mold cavity. In the course of making the mold, the mating faces of the mold segments are "spotted in" at the mold's operating temperature which involves finishing the complimentary faces such that there is a mating fit therebetween. Unfortunately, the "spotting in" process is not perfect, especially when forming molds have complex shaped mating faces. As a result, there is usually some degree of mismatch between the faces. This mismatching of the mold segments in turn results in the formation of small gaps between the mating faces at the parting line, and particularly at the edge of the parting line which is exposed to the mold cavity. When such gaps occur at the edge of the parting line, the liquid mixture injected into the mold cavity can invade the parting line at its edge and produce protrusions, known as "flash", on the finished article which must then be removed in a separate operation.
It is an object of the present invention to provide a substantially flash-proof reaction-injection-molding mold and process for making same.
This and other objects and advantages of the present invention will become more readily apparent from the following detailed description thereof.